1. Field of the Invention
The present invention relates to an image forming apparatus that uses an electrophotographic process such as in a copying machine, a printer or a facsimile, and more particularly relates to an image forming apparatus that can produce an output with a plurality of resolutions.
2. Description of the Related Art
For output devices using an electrophotographic process, it is increasingly required to enhance the image quality and the speed, and simultaneously, it is also required that a user can arbitrarily select the resolution according to the purpose, and for these applications, an image forming apparatus having a plurality of resolutions is used.
An electrophotographic process is performed as follows: light corresponding to image information such as an original document or electronic data is applied to the surface of an image carrying member charged by a charging device to form an electrostatic latent image, toner is adhered to the electrostatic latent image to form a visible toner image and the toner image is transferred to a sheet such as paper or photographic paper and is fixed to the sheet to obtain a predetermined image. Here, as a method of switching resolutions, there are known a method of switching the speed of a light change unit that writes the electrostatic latent image and a method of switching the speed of the image carrying member that receives light.
In the method of switching the speed of the light change unit such as a polygon minor, even when the resolution is switched, the speed of the image formation does not change, and even when a low resolution is selected, the print speed is the same as when a high resolution is selected. On the other hand, in the method of switching the speed of the image carrying member, when a low resolution is selected, it is possible to increase the print speed as compared with the case where a high resolution is selected, with the result that it is possible to meet a high-speed print application.
However, in terms of image quality, in the method of switching the speed of the image carrying member, since when a high resolution is selected, the speed of the image carrying member is lowered as compared with the case where a low resolution is selected, the time for development is increased. Consequently, there is a tendency that the density of an image is increased to increase its line width. Hence, when the resolution is switched, even for the same image, different images are disadvantageously output.
For this problem, a method of performing digital processing on image data to decrease the line width is known. A method of optimizing the conditions of image formation in the electrophotographic process to cope with the problem is also known. The amount of toner adhered is related to the characteristics of the image carrying member, the intensity of light in an exposure device, a potential difference between the bias voltage of a development device and the surface of the image carrying member and the like. Hence, for the problem described above, Japanese Unexamined Patent Application Publication No. 2001-265071 discloses that the image formation conditions are examined and that design conditions at the time of switching are indicated. Specifically, according to the sensitivity constant of the image carrying member, a relationship between the desired output of the exposure device and the bias voltage of the development device is indicated.
As a method of controlling the density and the line width of an image, there is a method of using a density sensor and a potential sensor, and a method of using the detection values thereof to control the exposure device and the development device is known. For example, Japanese Unexamined Patent Application Publication No. 2010-50639 discloses that a line drawing is formed, the width and the density thereof are measured and a development bias is adjusted and that a patch image is formed, the density thereof is measured and the exposure amount is controlled. Furthermore, Japanese Unexamined Patent Application Publication No. 2006-276164 proposes an another method because a sensor detects part of an image but it is difficult to detect the entire image formation region. In other words, data on the surface potential of the image carrying member, data on the image and data on the exposure level are used to simulate the profile of the electrostatic latent image, and the quality of the image produced according to the simulated electrostatic latent image is evaluated, with the result that the exposure device and the development device are controlled. Specifically, while the surface potential of the image carrying member by the charging device and the exposure amount by the exposure device, the bias voltage of the development device and the like are being changed, the simulation is repeated, and conditions under which the desired image quality can be obtained are found, with the result that the image formation is performed under such conditions.
In the electrophotographic process, edge processing is normally performed in order to enhance character reproducibility, specifically, the line width of characters. When toner is adhered to the electrostatic latent image on the surface of the image carrying member, the amount of toner adhered by the potential difference (which may hereinafter be referred to as a development gap) between the surface potential and the bias voltage of the development device. Here, in an edge portion of the image, an electrical field enters from its periphery, thus the toner is more likely to be adhered and the toner is adhered to the outside of the periphery of the range of the electrostatic latent image in the edge portion. Consequently, the line width of the character in the toner image is increased as compared with the character width of the electrostatic latent image.
For example, when an output is produced at a resolution of 1200 dpi, the character area of an output image is about 1.3 times as large as that of an image that undergoes the edge processing and that has a resolution of 600 dpi. This is a level at which it can be identified visually, and thus the output result disadvantageously differs depending on the resolution. Hence, the edge processing for thinning the edge portion is performed. For example, for an image where a white character is drawn within a black solid, the edge processing is performed to prevent the white character from collapsing.
The edge processing described above is intended to thin the edge portion of a character/line image, and is performed only for a character/line image. As images, there are a character/line image that is determined by two values and a figure/photo image that is formed with multiple values having a plurality of intermediate values. Since when the edge processing is performed on the figure/photo image, intermediate gradation densities are affected to degrade the image quality, the edge processing is not performed on the figure/photo image. Hence, in order to determine to which one of the images the input image data belongs, the image forming apparatus incorporates a region determination function.
The region determination function incorporates a plurality of line memories for properly determining the region. As the number of line memories is increased, the region to be determined is expanded, with the result that it is possible to enhance the determination accuracy and the determination processing speed. On the other hand, as the resolution of an image to be processed is increased, the amount of information per unit area of the image is increased. For example, an image of 1200 dpi has four times as much information as an image of 600 dpi based on simple calculation. Hence, when a high resolution is used, the amount of data to be processed is increased, and thus it takes more time to perform region determination processing than a case where a low resolution is used. In order to solve this problem, it is necessary to incorporate a larger number of line memories.
However, in terms of cost constraints, it may be difficult to incorporate the necessary number of line memories. In such a case, instead of the edge processing, a method of thinning processing is required. In the technologies proposed in Japanese Unexamined Patent Application Publication No. 2001-265071 and Japanese Unexamined Patent Application Publication No. 2010-50639 described above, there are problems to be solved in the thinning processing in terms of general versatility and accuracy to which setting conditions can be applied. The technology proposed in Japanese Unexamined Patent Application Publication No. 2006-276164 is not sufficient in terms of cost and standby time.
An object of the present invention is to provide an image forming apparatus that can achieve, without increasing the number of line memories, character reproducibility at a high resolution in the same manner as character reproducibility at a low resolution.